1. Field of the Invention
The present invention relates generally to an apparatus and method for reporting Channel State Information (CSI) in a wireless communication system, and more particularly, to an apparatus and method for reporting CSI for a stable operation of a network in a situation where a Precoding Matrix Indicator (PMI) and a Channel Quality Indicator (CQI) cannot be calculated.
2. Description of the Related Art
In the 3rd-Generation Partnership Project (3GPP) Radio Access Network 1 (RAN1), there are various feedback report modes for CSI a User Equipment (UE) feeds back with respect to a downlink channel state to an evolved Node B (eNB). The CSI is used for scheduling a downlink channel in the eNB.
Basically, there are two CSI reporting channels for a feedback report of the CSI; (1) a Physical Uplink Control Channel (PUCCH), and (2) a Physical Uplink Shared Channel (PUSCH). The CSI report is classified into a periodic CSI report and an aperiodic CSI report, and the periodic CSI report is done via the PUCCH and the aperiodic CSI report is done by the PUSCH.
The CSI, as is well known, includes at least one of a Rank Indicator (RI), a Precoding Matrix Indicator (PMI), and a Channel Quality Indicator (CQI). The RI indicates the number of ranks of a channel, i.e., the number of layers to be used for downlink transmission. The PMI includes information about a precoding matrix desired to be used for downlink transmission. The PMI is based on the number of layers indicated by the RI. The CQI indicates a modulation scheme and a code rate desired to be used for downlink transmission.
Further, the CSI report may be classified into a wideband report reflecting a state of a total cell bandwidth and a subband report reflecting a state of each subband.
Referring to the standard documents, i.e., 3GPP TS 36.211 v10.3.0 and 3GPP TS 36.213, v10.3.0, there are a plurality of modes for CSI reporting in the PUCCH, and among the plurality of modes, referring to PUCCH mode 2-1, when the transmission mode 9 with 8 CSI-RS is configured, the wideband/subband PMI and wideband/subband CQI are reported on a specific reporting time instance. The selected precoding matrix is selected by two PMIs, i.e., a first PMI and a second PMI.
The PMIs information and a single precoding matrix for a corresponding rank are selected according to a codebook, which is defined in Table 6.3.4.2.3-3 to 6.3.4.2.3-10 in the 3GPP TS 36.211 v10.3.0. The PMIs and the CQIs are calculated based on the last reported RI.
However, according to conventional standard techniques in association with calculation of the PMI and the CQI, the PMI and the CQI cannot be calculated in certain cases. In these cases, stable communication between a network and a UE cannot be guaranteed. More specifically, the wideband first PMI value is calculated based on the last reported periodic RI. The wideband or subband PMI is calculated based on the last reported periodic RI and the wideband first PMI. The wideband or subband CQI value is calculated based on the selected precoding matrix and the last reported periodic RI. When a reported RI is changed and the next coming report is a second PMI and CQI, there is no first PMI that is calculated based on the last reported RI. In this case, the second PMI and CQI value cannot be calculated. Therefore, a need exists for a scheme that provides stable communication between a network and a UE when the PMI and the CQI cannot be calculated.